<div><p>It was demonstrated previously that polar and non-polar surface extracts of the brown alga <i>Fucus vesiculosus</i> collected during winter from the Kiel Bight (Germany) inhibited bacterial attachment at natural concentrations. The present study describes the bioassay-guided identification of the active metabolites from the polar fraction. Chromatographic separation on a size-exclusion liquid chromatography column and bioassays identified an active fraction that was further investigated using nuclear magnetic resonance spectroscopy and mass spectrometry. This fraction contained the metabolites dimethylsulphopropionate (DMSP), proline and alanine. DMSP and proline caused the anti-attachment activity. The metabolites were further quantified on the algal surface together with its associated boundary layer. DMSP and proline were detected in the range 0.12–1.08 ng cm<sup>−2</sup> and 0.09–0.59 ng cm<sup>−2</sup>, respectively. These metabolites were tested in the concentration range from 0.1 to 1000 ng cm<sup>−2</sup> against the attachment of five bacterial strains isolated from algae and sediment co-occurring with <i>F. vesiculosus</i>. The surface concentrations for 50% inhibition of attachment of these strains were always <0.38 ng cm<sup>−2</sup> for DMSP and in four cases <0.1 ng cm<sup>−2</sup> for proline, while one strain required 1.66 ng cm<sup>−2</sup> of proline for 50% inhibition. Two further bacterial strains that had been directly isolated from <i>F. vesiculosus</i> were also tested, but proved to be the least sensitive. This study shows that DMSP and proline have an ecologically relevant role as surface inhibitors against bacterial attachment on <i>F. vesiculosus</i>.</p> </div
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